// typeof 遇到null 失效
// instanceof 不同iframe 和 window 间检测时失效
// Object.prototype.toString 遇到null undefined 失效
// constructor
// duck type

// typeof null === "object" ?
// obj instance Object

Object.prototype.toString.apply([]) === "[object Array]"
Object.prototype.toString.apply(function(){}) === "[object Function]"
Object.prototype.toString.apply(null) === "[object Null]"
Object.prototype.toString.apply(undefined) === "[object Undefined]"
